This invention relates to a desynchronizer for a synchronous digital communications system as set forth in the preamble of claim 1, and to a method of generating a jitter-reduced output signal from a synchronous digital input signal as set forth in the preamble of the claim 6.
Synchronous digital communications systems are based on recommendations of the ITU-T for SDH (synchronous digital hierarchy) or SONET (synchronous optical network). According to ITU-T Recommendation G.707 (03/96), page 6, message signals to be transmitted are packaged in virtual containers and nested in a synchronous transport module. The synchronous transport module is then transmitted. For each possible virtual container, a bit rate is defined in the Recommendation.
During transmission, low-frequency phase variations caused by temporary storage in different buffers of the communications system may add up. Such low-frequency phase variations are referred to as xe2x80x9cwanderxe2x80x9d and may result in buffers overflowing, so that data may be lost. One way of reducing or avoiding such wander is to use for the virtual container a slightly higher or lower bit rate than that specified in the Recommendation. By this measure, the fill level of buffers of intermediate network elements is shifted to the range of the lower or upper limit and held there, thereby reducing the adding up of wander. On the other hand, the number of pointer actions is increased in order to compensate for the difference in bit rate. This results in increased jitter.
From M. Klein and R. Urbansky, xe2x80x9cSONET/SDH Pointer Processor Implementationxe2x80x9d, 1994 IEEE GLOBECOM Communications: The Global Bridge, San Francisco, Nov. 28-Dec. 2, 1994, Conference Record, pp. 655-660, a synchronizer for SDH or SONET systems is known which produces a synchronous output signal from plesiochronous or pseudo-synchronous digital input signals. The output signal is transported through the communications system and fed at the other end to a desynchronizer to recover the original plesiochronous or pseudo-synchronous message signals. The desynchronizer includes a buffer in which the received signals are stored and from which they are read at a recovered clock rate to obtain the original message signal. As a measure for adjusting the recovered clock, the fill level of the buffer is used, which is determined from the write and read clocks of the buffer by means of a phase detector. Jitter is reduced by low-pass-filtering the recovered clock.
The desynchronizer known from Urbanskiy is not suited for effectively reducing the low-frequency jitter caused by pointer actions.
It is therefore an object of the invention to provide a desynchronizer for a synchronous digital communications system which is capable of effectively suppressing jitter caused by pointer actions. Another object of the invention is to provide a method of generating a jitter-reduced output signal from a synchronous digital input signal.